William A. M. Blumberg

Quenching of N2(a 1Πg, v’=0) by N2, O2, CO, CO2, CH4, H2, and Ar

We have determined quenching rate coefficients for N2(a 1Πg, v’=0) by N2, O2, H2, CO2, Ar, CH4, and CO using two‐photon laser excitation. Quenching by N2 appears to proceed via collisional coupling to N2(a’ 1Σ−u, v=0) with a rate coefficient of 2.2±0.2×10−11 cm3 molecule−1 s−1. Quenching by Ar is nearly as efficient. Gas‐kinetic rate coefficients are obtained for quenching by CO, O2, H2, CO2, and CH4. Collisional energy transfer from N2(a,0) to CO(A) is observed in these experiments. However, quenching by O2, H2, CO2, and CH4 is thought to be reactive.

NH (X 3∑−, v=1–3) formation and vibrational relaxation in electron‐irradiated Ar/N2/H2 mixtures

Measurements of the dynamics of NH(X3∑−, v =1–3), created in electron‐irradiated N2/H2 and Ar/N2/H2 mixtures, have been performed. Time‐resolved Fourier spectroscopy was used to observe NH(v→v–1) vibrational fundamental band emission. Time‐dependent populations were then determined by spectral fitting. Subsequent kinetic fitting of these populations using a single‐quantum relaxation model and a power‐law dependence of kv on v yielded the following NH(v =1–3) relaxation rate constants (units of 10−14 cm3 s−1): kv=1(N2)=1.2±0.5, kv=2(N2)=3.8±1.5, kv=3(N2)=7.5±2.5; kv=1(Ar)=0.2±0.1, kv=2(Ar)=0.5±0.2, kv=3(Ar)=0.8±0.3; kv=1(H2)≤50, kv=2(H2)≤100, kv=3(H2)≤150. In addition, the N2/H2 data provided a measurement of the nascent excited vibrational state distribution resulting from the reaction N(2D)+H2→NH(X,v)+H. The ratio NH(1):NH(2):NH(3) was found to be 1.0:0.97:0.81 (±0.28 in each value). Comparison of the observed nascent distribution with that of a statistical model suggests that the ratio NH(0):NH(1)=0.47....

Experimental determination of the Einstein coefficients for the N2(B–A) transition

We have used a branching‐ratio technique to measure the relative variation in the transition‐dipole moment with internuclear separation for the N2(B–A) transition. Our spectral observations cover the range from 500 to 1800 nm, and use several different detectors and excitation sources. The data from different sets are consistent in the regions of spectral overlap. Using well established values for the radiative lifetimes of N2(B,v’≥5) allows the relative dipole‐moment function to be placed on an absolute basis. From the dipole‐moment function and a set of RKR‐based Franck–Condon factors which we have computed, we derive Einstein coefficients covering the range v’=0–12 and v‘=0–20. Our results indicate that currently accepted lifetimes for N2(B,v’=0–2) should be revised upwards by 20% to 40%.

Analysis of hydroxyl earthlimb airglow emissions: Kinetic model for state‐to‐state dynamics of OH (υ,N)

Detailed spectroscopic analysis of hydroxyl fundamental vibration-rotation and pure rotation emission lines has yielded OH(υ,N) absolute column densities for nighttime earthlimb spectra in the 20 to 110-km tangent height region. High-resolution spectra were obtained in the Cryogenic Infrared Radiance Instrumentation for Shuttle (CIRRIS 1A) experiment. Rotationally thermalized populations in υ = 1–9 have been derived from the fundamental bands between 2000 and 4000 cm−1. Highly rotationally excited populations with N ≤ 33 ( ≤ 2.3 eV rotational energy) have been inferred from the pure rotation spectra between 400 and 1000 cm−1. These emissions originate in the airglow region near 85–90 km altitude. Spectral fits of the pure rotation lines imply equal populations in the spinrotation states F1 and F2 but a ratio Π(A′):Π(A″) = 1.8±0.3 for the Λ-doublet populations. A forward predicting, first-principles kinetic model has been developed for the resultant OH(υ,N) limb column densities. The kinetic model incorporates a necessary and sufficient number of processes known to generate and quench OH(υ,N) in the mesopause region and includes recently calculated vibration-rotation Einstein coefficients for the high-N levels. The model reproduces both the thermal and the highly rotationally excited OH(υ,N) column densities. The tangent height dependence of the rotationally excited OH(υ,N) column densities is consistent with two possible formation mechanisms: (1) transfer of vibrational to rotational energy induced by collisions with O atoms or (2) direct chemical production via H + O3 → OH(υ,N) + O2.

Formation and vibrational relaxation of OH (X 2Πi,v) by O2 and CO2

Time‐resolved OH(X 2Πi,v=1–9) populations have been measured and analyzed to determine parameters relating to formation mechanisms and vibrational relaxation. OH(v) was formed in electron‐irradiated Ar/H2/O3 mixtures containing added O2 or CO2 as relaxer species. OH(v→v−1,v−2) emission was observed using time‐resolved Fourier spectroscopy. Spectra were then fit to determine time‐dependent populations. Population data were analyzed using a single‐quantum relaxation model, but the possible effects of multiquantum relaxation were also considered. The model includes provision for OH(v) production via H+O3→OH(v)+O2 after e‐beam termination, which has been found to have a significant effect on the results. The following relaxation rate constants are obtained: kv=1–6(O2)=1.3±0.4, 2.7±0.8, 5.2±1.5, 8.8±3.0, 17±7, 30±15 (10−13 cm3s−1) and kv=1–4(CO2)=1.8±0.5, 4.8±1.5, 14±5, 28±10 (10−13 cm3s−1), respectively. Two different exponential decay rates are necessary to characterize the time dependence of the inferred H ...

Vibrational relaxation of OH(X 2Πi, v=1–3) by O2

Rate constants for OH(X 2Πi, v=1–3) vibrational relaxation induced by nonreactive collision with O2 have been measured. OH(v) is created by the H+O3 →OH(v≤9)+O2 reaction in an electron‐irradiated O3, H2, Ar mixture. OH(v) fundamental and first overtone IR emission is observed using time‐resolved Fourier spectroscopy. Spectral fitting followed by kinetic fitting of the resultant populations using a single‐quantum relaxation model yields rate constants of kv=1 =(1.3±0.4)×10−13, kv=2=(2.1±0.3)×10−13, kv=3=(2.9±0.8) ×10−13 (all units are in cm3 /s). Our measurements are consistent with and extend published results on the same system, as well as predictions made by Schwartz–Slawsky–Herzfeld theory.

N2+ Meinel band quenching

We have measured the rate coefficients for quenching the A 2Πu state of N2+ by air to be (7.0±0.4), (7.5±1.0), and (7.0±1.0)×10−10 cm3 molecule−1 s−1 for vibrational levels 2–4, respectively. Rate coefficients for quenching vibrational level 2 by molecular nitrogen and oxygen are (7.5±0.8) and (6.2±0.6)×10−10 cm3 molecule−1 s−1, respectively. Our results show that Meinel‐band quenching becomes significant at altitudes below100 km.

Branching ratios for infrared vibrational emission from NO(X 2Π,v’=2–13)

The ratios of overtone and fundamental vibrational Einstein coefficients for NO(X 2Π) have been measured by spectrally resolved infrared chemiluminescence near 2.7–3.3 μm and 5.2–6.8 μm. The reactions of N(2D,2P) with O2, in the presence of a small background of He in a cryogenic low‐pressure reactor, generated vibrationally excited, rotationally cold (60 K) NO(v), whose emission spectra were recorded with high spectral resolution. Least‐squares spectral fitting analysis of the observed overtone and fundamental spectra gave vibrational band intensities, whose ratios at each emitting vibrational level v’ yielded the (Δv=2)/(Δv=1) Einstein coefficient ratios for v’=2–13. The results provide comparisons to previous theoretical and experimental data, and reflect the behavior of the dipole moment function for NO(X 2Π). The measured ratios indicate an overtone Einstein coefficient A2,0=0.94±0.11 s−1 for an assumed fundamental value A1,0=13.4 s−1.

The radiative lifetime of N2(a 1Πg, v=0–2)

We have employed direct two‐photon laser excitation of specific vibrational levels of N2(a 1 Πg) to measure the lifetime of this state. Direct observation of emission from the a 1Πg –X 1∑+g transition in a large cell was employed to follow fluorescence decays. Experiments were conducted to verify that the effects of collisional transfer and diffusion were not contributing to the observed lifetime. Our experiments showed that the radiative lifetime of vibrational levels 0–2 is 56±4 μs and is independent of vibrational level, within experimental error. The observed lifetimes are in good agreement with recently reported theoretical calculations.

Velocity dependent O atom IR excitation cross sections : connections with flight data

A fast oxygen atom source has been used to study the velocity dependence of O atom infrared excitation reactions with various molecular species in a crossed beam experiment. These short wave infrared (SWIR) measurements are performed under single collision conditions, simulating the low Earth orbit environment. Such data are fundamental to the analysis and interpretation of atmospheric oxygen atom interaction with plume exhaust species and with the local environment about structures in low Earth orbit. Measurements have been performed over the oxygen atom velocity range of 6 to 12 km/s. These are the first such experimental measurements, and they may be used to validate theoretical estimates presently used in predictive models. We specifically discuss the velocity dependent cross sections for the reaction O + N2 → NO(v) + N. This reaction is endothermic for O atom velocities ≤ 8 km/s. Preliminary data are also shown for the reaction O + CO2 → CO2(ν3) + O → CO(v) + O2. Both band integral and spectrally resolved results will be presented. The spectrally resolved data provide information on the rotation/vibrational distribution of the excited states. Limited observations of infrared emissions resulting from atmospheric interactions have become available in recent years from both shuttle- and rocket-borne experiments. Comparisons between our experimental database and selected data from several different flight experiments are provided. These comparisons clearly identify the kinetic mechanisms responsible for the flight observations.